Reference is made to U.S. Pat. No. 4,592,882 where alumina-zirconium/Titanium or both diborides are prepared from Al2O3 and Ti/Zr diboride particulate mixing. The mixture is pressed and sintered as pressure-less or hot pressing or hot isostat pressing (HIP) at around 2000° C. This process is conventionally followed but the grain-growth of borides as well as alumina hinders the densification and are undesired. The processing time is longer and requires costly high temperature furnaces. Similar processing are also reported by T. Watanbe and S. Kouno, Bull. Am. Ceram. Soc., 61, 970, 1982 and G. Suskin, G. Chepovetsky, J. Mater. Eng. Performance, 5, 396, 1996.
Reference is also made to patent GB 2285267 where composite of zirconium diboride with zirconium oxide and other oxides, AlN, ZrN and BN have been reported. The product has used low melting point borides such as Ni/Co boride. These will lead to larger grain-growth as well as reduce the high temperature uses of such composite. The composite powder is prepared by Self-Propagating high temperature synthesis (SHS) process from elemental powders (K. Upadhya, J. Yang, W. Hoffman, Bull. Am. Ceram Soc., page 51, 1997 A. Makino, C. K. Low, J. Am. Ceram Soc. 77(3), 778, 1994. Also described in U.S. Pat. No. 5,061,662 by J Jung.) followed by milling, pressing and sintering at high temperatures in the range of 1900-2000° C. or hot isostat pressing at 1800° C. or more. This process also leads to coarser grains and costly high temperature furnaces to get a 95% or more dense materials. Beside these, the use of elemental powders makes the process cost intensive, also milling of the synthesised powder composite powder is required, which are very hard when processed through elemental stating powders. This step includes lot of impurity of the milling media during processing
It is also known to carry out preparation of alumina-Ti/Zr diboride composite by SHS process from their oxides and aluminothermic reduction (Jenq Liu and P. Darrellown by J. Am. Ceramic society, 1991, 74 (1), pp 241-43, V. Sundaram, K. V. Logan and R. F. Speyer; J. Mater. Res., 1997, 12, pp 1681-1684. S P. Ray: Metallurgical Transaction A, 1992, 23A, pp 2381-2385. S. Postrach, J. Potschke: J. Euro. Ceram. Soc., 2000, 20, pp 1459-1468). The prepared powders are milled and sintered at in the range of 1800-2000° C. or hot isostat pressing at 1800° C. or more. This process also leads to coarser grains and costly high temperature furnaces to get a 95% or more dense materials. However, the powders are loosely bound so milling is easy and hence the impurities are included to the lesser extent compared to particulate mixing process.
Reference is made to U.S. Pat. No. 4,647,405 which discloses the reaction synthesis and consolidation of ceramic composite of group IVA metal diboride-alumina. While the said patent reports a good dense body the composite had bulky agglomerates of alumina.